Field of the Invention
The present invention relates to a photoelectric conversion device and a manufacturing method thereof.
Description of the Related Art
As imaging apparatuses chiefly used in digital still cameras and video camcorders, demand for photoelectric conversion devices has been increased. Among photoelectric conversion devices used in digital still cameras, in particular those having a large size, such as APS-C size to 35 mm film size, have recently been widely used. As the above photoelectric conversion devices, CCD-type or MOS-type photoelectric conversion devices are used. Such a photoelectric conversion device includes a number of pixels arranged in an array across a large region, and a well contact is provided in every pixel in order to stabilize a reference potential at a center portion of a pixel circuit section and in a peripheral circuit section.
A method of forming a well contact for supplying a reference potential to a common well among pixels is disclosed in Japanese Patent Laid-Open No. 2006-73607. In the method disclosed in Japanese Patent Laid-Open No. 2006-73607, the common well among the pixels is formed as a p-type impurity region, and a p-type impurity region having a higher p-type impurity concentration than that of the common well is further formed in the well contact in every pixel prior to formation of a contact interlayer film. P-type impurity having a high concentration is further ion-implanted into an opening of a contact hole after the contact hole is opened, thereby forming the well contact having a higher concentration of the p-type impurity than that of a p-type impurity introducing region.
In the case of forming the well contact in this manner, contact etching and ion-implantation may induce crystal defects, or may cause contamination with impurity atoms of heavy metals, or the like. Crystal defects and contamination with impurity atoms of heavy metals in a well contact region may cause deterioration of sensor characteristics, particularly deterioration of noise characteristics, such as occurrence of white defects and dark currents.
Although not mentioned in Japanese Patent Laid-Open No. 2006-73607, it is necessary to supply a stable potential for contact holes to n-type impurity regions of a pixel circuit section, such as a source region, a drain region, a gate region, and a floating diffusion region. Therefore, after opening each contact hole in the pixel circuit section, n-type impurity having a higher concentration than that of p-type impurity should be ion-implanted into the opening of each contact hole. In this case, the n-type impurity is also ion-implanted in a well contact region to be a p-type impurity region, which causes change in effective p-type impurity concentration (a net p-type impurity concentration) distributions, so that it is concerned that white defects may occur, and dark current characteristics may become deteriorated. Hence, it is essential to control an impurity profile including not only concentration distributions of p-type impurity formed before the contact hole is formed, but also concentration distributions of n-type impurity to be ion-implanted, thereby controlling distributions of effective p-type impurity concentration.